Hibernate Automatic Dirty Checking

Hibernate persistent objects are keep synchronize with database while the session is remain open. But the data will not commit to the database unless we call session.flush() or committing transaction.

Hibernate tracks all persistent objects and it call update statements only for updated objects. This process of monitoring persistence objects and updating only modified object is called automatic dirty checking.

This process is leads to improve performance of updating database.

Hibernate Persistence Lifecycle

Hibernate persistence lifecycle has 3 states.

1. Transient
2. Persistent
3. Detached

Transient State

When create object using new operator, it is immediately goes to transient state. In this state the object is not connected with database and it is in memory. In this state if object is dereference, then it is goes to detached mode and garbage collector will clean it. To move object to persistent state you have to save this. If you create object from persist data, then the object is not in Transient state and it is in persistent state since the object is connected with database.

Persistent State

When the object is referred to the valid database row, the object is in persistent state. When we call the delete() method, the data in database will remove and object will move to transient state.

Detached State

The persistent objects are holding data even after the transaction is completed. These objects will be detached from database when we close the session. You can detached object by calling evict() method.

Sample Hbm File

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 2.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping>
<!--
This is a simplified version of the Event class used to
demonstrate proxies.
-->
<class name="Event" table="events">
<id name="id" column="uid" type="long" unsaved-value="null">
<generator class="assigned"/>
</id>
<property name="name" type="string" length="100"/>
<property name="startDate" column="start_date"
type="date"/>
<property name="duration" type="integer"/>
<many-to-one name="location" column="location_id"
class="Location"/>
</class>
</hibernate-mapping>

Sample Hibernate Config File

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
<session-factory>
<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="hibernate.connection.url">jdbc:mysql://localhost/hibernatetutorial</property>
<property name="hibernate.connection.username">root</property>
<property name="hibernate.connection.password"></property>
<property name="hibernate.connection.pool_size">10</property>
<property name="show_sql">true</property>
<property name="dialect">org.hibernate.dialect.MySQLDialect</property>
<property name="hibernate.hbm2ddl.auto">update</property>
<!-- Mapping files -->
<mapping resource="contact.hbm.xml"/>
</session-factory>
</hibernate-configuration>

Sample HQL

HQL example:

Query query = session.createQuery("from Table as tbl where tbl.column = :myId");

query.setParameter("myId", myId);

List result = query.list();

JDBC Connection Sample

package mycom;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.sql.Statement;

import com.mysql.jdbc.ResultSet;

public class testConnection {


public static void main(String[] args){

Connection conn = null;

try{

Class.forName("com.mysql.jdbc.Driver").newInstance();
conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/test","root","root");

if(conn==null)
System.out.println("Connection is null");
else
System.out.println("Connection = "+conn.toString());


java.sql.ResultSet rs = null;
Statement stmt = conn.createStatement();

rs = stmt.executeQuery("select * from country order by 1");

while(rs.next()){
System.out.print(rs.getString(1)+" | ");
System.out.println(rs.getString(2));
System.out.println("-----------------------------------");
}

}catch(SQLException e){
e.printStackTrace();
}catch (Exception e) {
e.printStackTrace();
}finally{
try {
conn.close();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}




}



}

What is Hibernate Session

Session is hibernate construct which is used to mediate connection with database.

At the time session is created, it is creating connection with database (Hibernate is use configurations given by configuration file to make the connection). Then session is holding connection until session ends. All objects loaded by Hibernate are associated with session. When session is ends Hibernate will automatically close the connection.

To use Hibernate persistence mechanism you have to initialize the Hibernate environment and then need to get session object from SessionFactory class. I'm showing below how to obtain session from SessionFactory .

// initializing Hibernate environment
Configuration cfg = new Configuration().configure();

// create session factory
SessionFactory factory = cfg.buildSessionFactory();

// getting the new session object
Session session = factory.openSession();